Drilling with downhole motors is well-known. A downhole motor assembly generally comprises of the hydraulic power section, a universal joint, and a bearing assembly. The nonsealed bearing assemblies are more and more being replaced by sealed bearing sections. The sealed bearing assembly contains radial bearings to guide the bit with respect to the drillstring and thrust bearings to transmit down-load and up-load thrust. These seal bearing assemblies are placed in housings which are filled with bearing lubricant, which is pressure-balanced to the drilling fluid pressure by virtue of use of pressure-transmitting partitioning devices such as free-floating pistons. As the mud motor assembly operates, the circulating mud acts as a coolant to the lubricant oil which is separated from the circulating mud by the seal assemblies, with the piston acting as the barrier between the lubricant and the circulating mud. By some designs, a certain amount of lubricant passes through the seals during operation. This leakage flow is important to the long-term vitality of the seals, which in turn keep the radial and thrust bearings of the bearing assembly in an oil bath during the drilling operations.
An alternative has been to use drilling mud-lubricated bearings. Because the mud has grit and other solids in it, these bearings were only run about 80-120 hours before the assembly was taken out of service. Wear in these bearings enlarged their clearances around the drive shaft and made the bit difficult to control because of its eccentric movements in enlarged bearing clearances.
In the past, drilling personnel have operated on a rule of thumb where after so many hours of operation on a downhole motor assembly, the assembly was withdrawn from the wellbore and the bearing section disassembled so that the lubricant could be replenished. In general, after approximately 80-120 hours of operation, drillers would pull the drilling motor assembly out of the hole in an effort to ascertain the amount of lubricant remaining. The techniques that were available for actual measurement of the remaining lubricant were either crude and always indirect or, in some sizes, completely not workable. Some of these techniques involved the installation of a pressure gauge once the bearing assembly was out of the hole. The gauge was generally screwed onto a check valve which communicated with the lubricant reservoir. A spring acting on the floating piston would produce a pressure which would be reduced as fluid escaped from the reservoir around the seal. Thus, based on the pressure reading obtained, an indirect measurement of the remaining fluid was obtained.
These devices were generally unavailable in smaller sizes due to space limits which precluded the insertion of a check valve to facilitate the insertion subsequently of a pressure gauge. Even on larger sizes, this technique proved cumbersome, and generally required the removal of the bearing assembly from the rig floor, thereby necessitating additional delays at the drillsite.
Other techniques that were used involve the insertion of a thin rod to, in essence, feel the position of the seal through a small opening available around the drive sub. This technique was only feasible if a compensating piston assembly was used. Again, this technique was unavailable on small sizes and was also inaccurate in determining the true volume of lubricant remaining in the reservoirs around the radial and thrust bearings.
In view of the uncertainties of these indirect techniques in measurement of remaining lubricant, drillers have opted for the replacement technique which involved removal of the downhole motor assembly and a replenishment of the lubricant level after approximately only 80-120 hours of operation. Accordingly, it is the object of the invention to provide improved sensing techniques for the lubricant level in a bearing assembly for a downhole motor drilling assembly so that the level of remaining lubricant can be easily determined by rig personnel at the surface, having removed the assembly from the wellbore, or alternatively, on a real-time basis by sensing the fluid level and transmitting the level remaining information to the surface during the drilling operations. It is estimated that with these improvements to measuring techniques for lubricant levels, the downhole motor assembly can be run continuously in the wellbore for 300 hours and higher prior to removal for a check on the condition of the fluid reservoir level. Of course, other downhole conditions may necessitate the removal of the mud motor drilling assembly earlier. However, it is intended with this invention that rig personnel can reliably continue to operate a downhole motor drilling assembly well over the pre-existing 80-120-hour arbitrary limit as long as the suitable rate of penetration is being maintained. The details of the operation of the apparatus and method will become clear to those of ordinary skill in the art from a review of the detailed description below.
The basics of use of a combination of a floating piston with a seal, coupled with a reservoir in a drilling mud motor seal bearing assembly is described in detail in U.S. Pat. No. 5,195,754. U.S. Pat. No. 4,610,319 also illustrates the use of a hydrodynamic lubricant seal in a drillbit environment.